Platinum submonolayers

Sasaki K, Zhang J, Wang J, Uribe F, Adzic R, Sasaki K, Zhang J, Wang J, Uribe F, Adzic RR (2006) Platinum submonolayer-monolayer electrocatalysts an electrochemical and X-ray absorption spectroscopy study. Res Chem Intermediate 32 543-559... 

Promotion and deactivation of unsupported and alumina-supported platinum catalysts were studied in the selective oxidation of 1-phenyl-ethanol to acetophenone, as a model reaction. The oxidation was performed with atmospheric air in an aqueous alkaline solution. The oxidation state of the catalyst was followed by measuring the open circuit potential of the slurry during reaction. It is proposed that the primary reason for deactivation is the destructive adsorption of alcohol substrate on the platinum surface at the very beginning of the reaction, leading to irreversibly adsorbed species. Over-oxidation of Pt active sites occurs after a substantial reduction in the number of free sites. Deactivation could be efficiently suppressed by partial blocking of surface platinum atoms with a submonolayer of bismuth promoter. At optimum Bi/Ptj ratio the yield increased from 18 to 99 %. 

In this paper we report the application of bimetallic catalysts which were prepared by consecutive reduction of a submonolayer of bismuth promoter onto the surface of platinum. The technique of modifying metal surfaces at controlled electrode potential with a monolayer or sub-monolayer of foreign metal ("underpotential" deposition) is widely used in electrocatalysis (77,72). Here we apply the theory of underpotential metal deposition without the use of a potentiostat. The catalyst potential during promotion was controlled by proper selection of the reducing agent (hydrogen), pH and metal ion concentration. 

Platinized platinum catalysts were modified by submonolayers of lead, deposited and characterized by electrochemical techniques. Their activity was tested in liquid phase hydrogenation of C C bonds. 

Noble metal electrodes include metals whose redox couple M/Mz+ is not involved in direct electrochemical reactions in all nonaqueous systems of interest. Typical examples that are the most important practically are gold and platinum. It should be emphasized, however, that there are some electrochemical reactions which are specific to these metals, such as underpotential deposition of lithium (which depends on the host metal) [45], Metal oxide/hydroxide formation can occur, but, in any event, these are surface reactions on a small scale (submonolayer -> a few monolayers at the most [6]). 

Conductive anodic silicon oxide films can be produced by doping a submonolayer of platinum in the oxide. ° The platinum is deposited on the silicon surface gal-vanostatically from 5% H2PtCl6 for a thickness from 0.002 to 2.5 monolayers. The silicon is then anodized in 0.2 M H2SO4 under illumination followed by a heat treatment. The Pt is present in the film at 0.25 (at Si/Si02) to 0.03 (SiOa/solution) atom % and either as Pt or as PtO with an energy level close to the n-Si valence band edge. 

Hydrous oxides are of major interest in many areas of technology, e.g., corrosion and passivation of metals, formation of decorative, protective, and insulating films, aqueous battery systems, catalysis and electrocatalysis, electrochromic display systems, pH monitoring devices, soil science, colloid chemistry, and various branches of material science. Detailed accounts of some of the nonnoble hydrous metal oxide systems, especially aluminum,1 have appeared recently. In the case of the noble metals such as platinum or gold most of the electrochemical work to date has been concerned with compact monolayer, and submonolayer, oxide growth. 

The investigation on the electrochemical epitaxial growth of palladium on gold and platinum singlemonolayer coverage of palladium on Pt(lll) electrode by the immersion technique for the first time [74]. Llorca and coworkers investigated the irreversibly adsorbed palladium on Pt(hiJ) in acidic solution [75] and reported that electrocatalytic activity for the oxidation of formic acid on the Pt(lOO) electrodes vras improved drastically by the palladium-adlayer modification, while that on the Pt(lll) electrode was not greatly affected by the Pd-modification [76]. However, it is difficult to prepare an ultrathin film of palladium with various thickness by the immersion technique, and STM observation of the atomic structure of these surfaces is not available yet. 

The rate and efficiency of crystallization of calcium gluconate from solution following the electrode oxidation of glucose have been investigated, and this reaction at a platinum electrode is markedly catalysed by submonolayers of heavy metals (e.g, thallium,... 

The development of new electrocatalytic materials by deposition of foreign atoms on a host metal is currently a very active area of research in electrochemistry. In this regard, most of the studies have been focused on platinum as a substrate, due to the very high reactivity of this metal for fuel-cell reactions. Still, a surface modifier is necessary to improve the catalytic activity and stability of the electrode and to avoid side poisoning reactions. It has been shown that the modification of platiniun surfaces with submonolayer amounts of elements of the p-block of the periodic table leads, in many cases, to electrode materials with greatly improved catalytic properties. ... 

The PtML/Ru electrocatalysts were inaugurated for fuel cell anode reactions, and the catalysts were synthesized by two methods. The first method facilitated the formation of submonolayer-to-multilayer Pt deposits on Ru surfaces via the electroless (spontaneous) deposition of Pt on Ru [103-107]. The coverage and morphology of the Pt deposit on Ru depended on the concentration of platinum ions and the time of deposition. The activity and selectivity of the electrocatalyst was fine-tuned by changing the coverage (the cluster size) of the Pt deposit, and the optimized PtRu2o/C (with atomic ratio of Pt Ru as 1 20) electrocatalyst demonstrated superior CO tolerance and stability compared to conventional Pt-Ru/C alloy catalyst [104]. 

A Knudsen oven and quartz crystal thickness monitor have been attached to a HPLP system, enabling accurate, reproducible dosing of submonolayer quantities of gold under the ultraclean conditions of the UHV chamber. Results indicate that atoms of gold inhibit the catalytic activity of the platinum in heptane isomerization. However, annealing to form a 2-D alloy (and the consequent separation of surface platinum into small ensembles), actually led to an increase in isomerization activity over the dean platinum value. 

Ad-atom Effects on Formic Acid Oxidation Submonolayers of some metal atoms, irreversible adsorbed or under-potentially deposited (upd) [89] on a metal substrate, can present electrocat-alytic properties different from those of the pure metals. They may be similar to those of alloys or codeposited metals [90]. Typical examples are submonolayers of As, Sn, Bi, H, and Pb on platinum [91-93]. Since a upd layer can exist within a given potential range only, this modified catalyst may not fulfill the conditions for technical application. 

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